As is well known, there are many examples of dual-element fuse links with at least one of the elements being utilized to open the fuse when there is a short circuit and another element being utilized to open the fuse when there is an overload.
The dual-element fuse normally has a mass of heat-softenable alloy which is in contact with a heat-absorbing element so that the softening of the mass of heat softenable alloy is delayed to provide a desired-predetermined, time-delay before the electric fuse responds to a relatively low, but potentially-harmful over current to effect opening of the circuit.
In some instances, the overload fuse portion is operated by a spring so that when the heat softenable material melts, the spring quickly opens the circuit. In these instances, the overload fuse element is usually maintained in air and is separated from any quartz sand or other material which is in the fuse element.
In other time delay fuses, the overload current is provided by a solid mass which is surrounded by sand. The sand acts as a heat conductor and allows the solid mass to melt. However, in such an instance, there are many times when the fuse does not blow due to the melting mass and sand acting as a conductor. These types of fuses are shown in U.S. Pat. No. 4,417,224, the Aeroflex Publication, U.S. Pat. No. 4,973,932. The reliability of these types of fuses while generally acceptable, still need improvements.